Photosynthesis• 6 CO2 + 12 H2O --> C6H12O6 + 6 O2 + 6 H2O

• carried out by photoautotrophs• Solar energy --> chemical energy

• redox process- water oxidized, CO2 reduced

• Photosynthesis occurs in 2 stages:• 1. light reactions (photo)• 2. Calvin cycle (synthesis)

• CO2 and O2 enter and leave through stomata

• Chloroplasts• found in mesophyll• contain chlorophyll = green pigment• thylakoid membranes in stacks called grana• in prokaryotes, thylakoids on cell membrane

• In each chloroplast are functional groups called

photosynthetic units• 1. contain about 300 molecules of

pigment• 2. contain one specialized chlorophyll a

molecule called the reaction center• 3. other molecules function like antennae• 4. reaction center can trap energy and

pass it along an enzyme-linked series of reactions to convert energy to a

usable form

• chlorophyll a- • 1. absorbs violet, blue-violet, red• 2. reflects green, yellow, orange

• carotenes- • 1. absorb wavelengths not absorbed by

chlorophyll• 2. provide photoprotection

•**All pigments absorb different wavelengths!**

• electrons within chlorophyll are excited to higher energy levels by photons (particle of light)

• What happens to excited electron?• 1. energy dissipated as heat• 2. energy reemitted as light

=fluorescence (in test tube)• 3. energy may cause a chemical

reaction (in chloroplast)

1. Light Reactions

• Light energy drives the transfer of e- and H from water to NADP+ ---> NADPH

• H2O is split and O2 is given off.

• Light reactions also carry out photophosphorylation converting ADP to ATP.

• Ground state e- + photon ---> excited e-• unstable• drops back • energy can be trapped by ETC

• Photosystem I= P700

• Photosystem II= P680

• Have identical chlorophyll a but different associated proteins

Noncyclic Photophosphorylation

• •occurs in green plants and a few bacteria• 1. Photon strikes P680 reaction center.• 2. Electron passed down ETC to P700.• 3. Some energy in chain used to make ATP

by chemiosmosis across the thylakoid membrane.

• 4. Electrons in P680 replaced by splitting H2O.• ** This is the step where O2 is released!**

• 5. Photon strikes P700 reaction center.• 6. Electrons passed through a second ETC.• 7. More ATP is made by chemiosmosis.• 7. As e- pass down ETC, NADP+ is reduced ->

NADPH.• **NADPH and ATP go on to the Calvin cycle.**

Cyclic Photophosphorylation

• •less efficient

• •only uses P700

• 1. Photon strikes P700 reaction center.

• 2. Electrons passed down ETC back to P700.

• 3. Generates ATP.

• 4. No production of NADPH or release of O2.

• ***Like oxidative phosphorylation in the mitochondria!***

• Cyclic Photophosphorylation continued…

• May have been the earliest form of ATP production.

• Still used by photosynthetic bacteria.

• Operates along with noncyclic flow in plants to generate more ATP. (The Calvin Cycle uses more ATP than is produced by

noncyclic photophosphorylation.)

2. Calvin Cycle

• •Doesn’t require light directly• (occurs during daylight for most plants!)

• Begins with Carbon fixation= Carbon from CO2 is incorporated into organic molecules

• 3 CO2 enter per cycle

• •The Calvin Cycle then reduces the carbon into carbohydrate by the addition of e- from NADPH

• •produces one 3C sugar (PGAL or G3P) per cycle

• Process: (for each CO2)• 1. CO2 attached to a five C sugar (RuBP)• -catalyzed by enzyme rubisco • (most abundant protein on Earth!!!)

• 2. Forms unstable 6-C intermediate which splits into 2 3-C sugars.

• 3. 3-C sugars phosphorylated by ATP.

• 4. 3-C sugars reduced by NADPH.

• 5. PGAL produced.

• 6 PGALs are produced for each Calvin Cycle• -1 exits cycle• -5 remain in cycle---> regenerate RuBP

• 6. Rest of cycle= regenerate RuBP (ATP needed)

• It takes 2 cycles to produce one glucose!

Special Cases of Photosynthesis

• Most plants= C3 plants

• CO2 fixed by rubisco & first product = 3C sugar

• In dry, hot weather, plants close stomata to reduce transpiration --> Reduces CO2 intake.

• Rubisco binds to O2 when [CO2] is low.

• =photorespiration- decreases photosynthesis

• C4 plants- form a four C compound as first product• ex- sugar cane, corn & some grasses• Krantz Anatomy- 2 types of photosynthetic cells• a. bundle sheath cells tightly packed around

veins• b. mesophyll cells located outside bundle

sheath• CO2 is fixed in the mesophyll by the enzyme

PEP carboxylase --> 4C compound • 4C compound then enters bundle sheath where

CO2 is released and is used by rubisco in the Calvin cycle.

• ***C4 pathway minimizes photorespiration and enhances sugar production!***

• CAM plants- no special cells, CO2 is trapped during the night (stoma open) for use during the day(stoma closed)

• incorporate CO2 into organic acids in night

• ex- cactus & pineapple